fixes to buffer copy! return an event object instead of dst for profiling
fixes to buffer copy! return an event object instead of dst for profiling

yuyichao/OpenCL.jl,yuyichao/OpenCL.jl,tkelman/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl,tkelman/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl